Cable-driven transmissions may be used to transmit motion (typically rotation) from one location to another in many applications, including mechanical, robotic, and mechatronic applications. Given their small cross-sectional diameter, cables can be routed through any general circuitous path without taking up too much space. Some common applications that employ cable based transmission systems include consumer products such as printers and photo-copiers as well as industrial systems such as gantries and material handlers. Further applications include robotic systems, medical devices, and other remote access devices.
Cables may include wires, ropes, strands, filaments, lines, tendons, pull-wires, etc. Cable driven transmission systems may typically rely on the tension that can be generated in a cable and transmitted via the cable. An example of a cable based transmission is shown in FIG. 1A. The driven pulley and the driving pulley are connected by two cables 307, 309 for bi-directional control (i.e. clockwise as well as counter clockwise). Depending on the actual construction, these two cables could be two distinct cables or a single cable that is continuously wrapped around both pulleys (e.g., having ends of the single cable that are interconnected). If the transmission is such that a pulley has to turn only over a partial rotation, then the cable may be locked to the pulleys via an engagement coupling such as a ferrule or crimp to avoid slippage.
When the driving pulley rotates counter clockwise (CCW) as shown in FIG. 1B, a tension may build on the left side cable 307 (which may be referred to as the transmitting cable). This tension may cause the driven pulley to also follow in the CCW direction. If there were no stretch in the cable, then the amount of cable pulled by the driving pulley on the left side 307 would be exactly equal to the amount of cable it releases on the right side 309. This would be considered a kinematically determinate transmission, and implies that the rotation of the driven pulley for a given rotation of the driving pulley is entirely dictated by the geometry of the overall system.
In reality this is generally not the case, as there is always some stretch in the region of cable on the left 307 because it is under tension, and because the cable has elasticity (i.e. it has some finite compliance along its length). Thus, the amount of cable pulled by the driving pulley on the left side may not be exactly equal to the amount of cable it releases on the right side. This results in the length of cable on the left side 307 being taut and the length of cable on the right side 309 being slack, as shown in FIG. 1B. In this example, there is no tension in the right side length of cable 309, and this transmission is no longer purely kinematic, because the rotation of the driven pulley for a given rotation of the driving pulley is dictated not only by the overall geometry of the system but also by the elastic properties of the cable. As a result, the driven pulley is no longer completely engaged with the driving pulley. For example, referring to FIG. 1B, if while holding the driving pulley fixed, if the driven pulley is perturbed in the clockwise direction 355, it would feel stiff. In other words, the driven pulley's rotation is determined or secured. However, if the driven pulley is perturbed in the CCW direction 357, it would feel compliant. Thus, the driven pulley rotation is not fully determined or secure. This lack of complete determinism of the driven pulley rotational position (which is sometimes also referred to as backlash, play, or slop) is a major drawback of cable driven transmission systems. Thus, there is a need for cable slack or tension management in cable driven transmission systems.
The direction of rotation of the driven pulley does not always have to be same as the direction of rotation of the driving pulley. The cable may be arranged in a slightly different manner, as shown in FIGS. 2A and 2B, such that a CCW rotation of the driving pulley produces a CW rotation of the driven pulley. Nevertheless, the above described problem of cable slack applies in this case as well.
In certain applications, this slack generation happens not just because of the elastic stretch of the cable, but also due to the geometry or kinematics of the transmission system. For example, in certain applications, mechanical and robotic tools often employ a controllably bendable elongate member. Such elongate members may be bent or articulated by cables, tendons, or pull-wires, and may allow bending in multiple directions. Unfortunately, when multiple cables (which may equivalently be wires, ropes, strands, filaments, lines, actuating cables, tendons, pull-wires, etc.) are used, bending in one direction, e.g., by pulling on one or more of the cables, may result in slack forming in the other cable(s). This slack may negatively impact the operation of the device, particularly when pulling/pushing the cables to control bending of the member, resulting in non-deterministic or unpredictable motion characterized by compliance or backlash.
One example of a controllably bendable elongate member includes a multi-link snake-like joint (also referred to as an end effector joint) that may provide articulated motion (i.e. wrist-like dexterity, or rotation about two axes) to an end effector. Bending movements may be controlled by two or more cables (including pairs of cables) coupled to some or all of the links. The end effector might include one or more of a grasper, probe, pliers, mini-scissors, light source, catheters, etc., in medical and non-medical tools. Such tools may benefit from a large angular range of rotation at the end effector region to provide reach and work space. Depending on the application, the end effector may articulate only in one direction (e.g. pitch or yaw) or in two orthogonal directions (e.g. pitch and yaw).
An example of an end effector joint design that allows two orthogonal rotations is a multi-link end effector joint which comprises links, or disks, or elements, or link elements with an alternating sequence of pivots to provide the two desired wrist-like rotations (e.g., pitch and yaw), and is illustrated in FIG. 7. In this example, a pair of cables (719, 719′ and 721, 721′) is used per each end effector rotation direction, and these cables pass through holes 729, 731, etc. on the periphery of the links, resulting in an end-effector joint that has two rotations (e.g., pitch and yaw). End effector joints such as this that include multiple serially linked elements may be maneuverable like a snake; hence they may be sometimes referred to as “snake-like joints”. Similarly, when link elements are serially connected via single-axis pivots (all aligned in one direction) only, the resulting device has an end effector rotational capability in a single plane, as illustrated in FIG. 8A. In this example, the “base link” 203 is attached to the tip or end of the tool (e.g., a medical device or a robotic arm). The “end link” 205 may be connected to an end effector (e.g., grasper, light, etc.) to provide the end effector with the desired articulation capability. The driving cables such as 719, 719′, 721, and 721′ are attached to/terminated at the end link but pass through the holes in the remaining links.
FIGS. 8A and 8B illustrate the problem with slack in the cable referred to above. When the elements or links forming the end effector device are serially linked via single-axis pivots all aligned in one direction only, the end effector rotation is limited to a single direction (pitch or yaw). Alternatively, when elements forming the articulating device are serially linked via pivots with alternating rotational axes perpendicular to each other, an end effector rotation capability may be possible in two rotational directions (pitch and yaw), as was shown in FIG. 7. Alternatively, elements forming the end effector joint may be serially linked, e.g., via ball and socket joints, which then provide three rotations (pitch, yaw, and roll) of the end effector. In all these cases, the end effector joint poses several design challenges. As an example, for rotation in any given plane (e.g. as shown in FIGS. 8A and 8B), it may be desirable to employ multiple links and pivots because this allows for a large range of rotational motion of the end link and in the associated work space. Also, by using multiple links and pivot joints, the rotation per link/joint is reduced, potentially resulting in a design that is more practical to manufacture and assemble, and that may operate at a smaller size scale. However, this design is inherently prone to problems associated with slack generation in the cables that are used for driving the end effector rotation discussed above. As shown in FIGS. 8A and 8B, upon rotating the pulley to operate on the first cable 207 to bend the device towards left, the geometry or kinematics of articulation is that the amount of cable 207 released on the left side may be greater than the amount of cable 209 pulled in on the right hand side, even when the cable elastic stretch is ignored.
When such a multi-link end effector joint is actuated by a driving pulley (either manually, mechanically, or via a motor, etc.) on the input end (or equivalently, the master end, user end, or control end) of an associated transmission cable, this typically produces a situation where the cable 307 on the left hand side remains taut while the cable on the right hand side 309 develops slack, as illustrated in FIGS. 9A and 9B. This situation may occur when the driving pulley rotates counter clock wise (CCW) and the end effector rotates to the left, as illustrated in FIG. 9B. This situation may be reversed when the end effector is actuated towards the right (i.e. rotating the pulley to turn clock-wise), and the right hand side cable 309 would be taut and the left hand side cable 307 will have slack.
In general it would be desirable to provide a simple solution to this problem, and particularly to provide a mechanical solution that does not require the use of additional sensors or powered actuators such as electric, pneumatic, or hydraulic motors or electronic/computer control. Thus, described herein are apparatuses, including tensioning pulleys incorporating and/or integrated with a slack take-up element that may address the issues raised above in cable-based transmission systems that may have slack due to either geometric/kinematic reasons (e.g. in case of driving a multi-link end-effector joint) or due to elastic reasons (e.g. compliance of cable and/or other transmission elements).